There are two categories of diaphragm pump: pumps in which the diaphragm is actuated hydraulically, and those in which the diaphragm is actuated mechanically. In hydraulically-actuated pumps, a reciprocating piston acts in a chamber containing a determined volume of "drive liquid", with one of the walls of the chamber being constituted by the diaphragm to be actuated. Pushing the piston into the chamber causes the diaphragm to be pushed back into it working chamber whose volume is thus decreased. This constitutes the pump delivery stroke. In its reverse stroke, the piston establishes suction in the control liquid which pulls back the diaphragm. The volume of the working chamber increases. This is the pump suction stroke. The strength of the suction in this type of pump is limited by cavitation in the working liquid.
In a pump whose diaphragm is actuated mechanically, the diaphragm is coupled to reciprocating moving equipment. Several drive mechanisms exist for the moving equipment, which mechanisms are of the crank and connecting rod type or rather of the type comprising a slider coupled to an eccentric. In some of them, the eccentric acts like a cam which drives the moving equipment (during the pump delivery stroke), with return being provided resiliently. In others, the moving equipment is coupled to the eccentric via a coupling nut providing go and return drive.
The flow rate of such diaphragm pumps is adjusted by acting on two operating parameters: rate; and stroke amplitude. In practice, the rate is acted on by adjusting the speed of the motor driving the eccentric. The amplitude of the stroke is adjusted by mechanisms that depend on pump technology. Thus, for hydraulically-controlled pumps, for constant amplitude of the piston stroke, it is possible to adjust the quantity of control liquid that is displaced. To do this, the control chamber is contained in part in a cavity in the piston, which cavity includes lateral orifices in communication with a tank, said orifices being open over an adjustable fraction of the stroke around the bottom dead center position of the piston (at the end of the suction stroke). An example of this technique is illustrated in Document EP 148 691.
Otherwise, for mechanically-actuated pumps, the stroke is generally adjusted by limiting the return amplitude of the slider under drive from the return spring by means of an adjustable abutment, as described, for example, in Documents: U.S. Pat. No. 4,167,896 or GB-A-2 044 895.
There is no advantageous solution for adjusting the amplitude of the stroke when suction is achieved by the moving equipment being positively driven by the eccentric.
In some markets, such as treating waste water in particular, hydraulically-actuated pumps are still seen as being complicated devices requiring expensive monitoring and maintenance. In addition, users always fear that a rupture of the diaphragm will lead to the treated liquid mixed with the control fluid (oil) with severe pollution consequences. There exists a remedy for this risk which consists in installing two diaphragms together with a rupture detection device, but in the eyes of users used to simpler equipment, this merely complicates the apparatus.
The present invention is a response adapted to the state of the market, i.e. a mechanism for adjusting the flow rate of a mechanically-actuated pump presenting the same advantages as a hydraulically-controlled pump with respect to ease of adjustment and retaining pumping characteristics regardless of flow rate.